In this project we will investigate a nonhybrid Xiphophorus model of spontaneous melanoma formation and susceptibility using behavioral and genetic analyses. X. cortezi is polymorphic for the Xmrk oncogene which is the result of a recent duplication of the mammalian epidermal growth factor receptor gene (egfr). The continued nonfunctionalization of Xmrk within this divergent genus, despite its deleterious association with malignant melanomas, has yet to be explained. This proposal outlines four research projects designed to address the potential selective maintenance of Xmrk through its intimate linkage with the Mdl (macromelanophore determining locus). This genus relies on melanin visual signals in kin recognition and mate selection. Some of the melanin signals encoded by the Mdl locus become cellular progenitors of the neoplasm at the onset of tumor induction. Previous work in our lab has shown that females of X. cortezi prefer males with more pigment to males with less. Thus, the potential for selective pressure exists within pigment alleles encoded by the Mdl locus. Selective pressure for Mdl alleles used in mate choice could supersede the deleterious nature of the linked Xmrk locus providing its evolutionary conservation over time.